1. Technical Field of the Invention
The present invention relates to a detection apparatus, and in particular to a detection apparatus that detects an amount of particulate matter in an exhaust gas that flows through the exhaust path of an internal combustion engine.
2. Related Art
Recently, internal combustion engines are required to have superior exhaust purification performance. In diesel engines, in particular, removal of so-called exhaust particulates (particulate matter (PM)), such as black smoke, exhausted from the engines is of increasing importance. In order to remove PM, diesel engines are most commonly equipped with a diesel particulate filter (DPF) in the middle of the exhaust pipe.
PM sensors are one of the means for detecting the amount of PM in an exhaust gas. For example, using a detection value derived from a PM sensor disposed downstream of a DPF, a fault of the DPF, if any, can be detected. Further, when such a PM sensor is disposed upstream of a DPF, the amount of PM accumulated in the DPF can be estimated from a detection value derived from the PM sensor. For example, JP-A-S59-060018 discloses a system for estimating the amount of PM accumulated in a DPF by disposing a PM sensor in an exhaust pipe.
As a matter of course, a PM sensor is desired to have high accuracy when used for detecting a fault of a DPF or estimating the amount of PM accumulated in the DPF. However, detection values of a PM sensor depend such as on the operating conditions of the engine and thus high accuracy is not ensured in the detection.
As shown in FIG. 3, a PM sensor 5 of a typical structure includes an insulator 50, a pair of electrodes 51 and 52, and a power supply 54. When the PM sensor 5 is disposed in an exhaust pipe through which PM flows, PM is deposited on the insulator 50. Since PM is electrically conductive, accumulation of PM between the electrodes 51 and 52 to the extent of connecting therebetween will create an electrically conductive state across the electrodes. Accordingly, when voltage is applied across the electrodes 51 and 52 by the power supply 54, current passes across the electrodes 51 and 52. As more PM is accumulated between the electrodes 51 and 52, more current passes across the electrodes. Therefore, the amount of PM accumulated on the insulator, and further, the amount of PM in the exhaust pipe is detected (estimated) based on the current passing across the electrodes.
The inventors of the present invention have expertise that detection values of a PM sensor depend such as on the temperature of and around the PM sensor, the exhaust flow rate around the PM sensor and the output of the PM sensor. In a condition where different temperature regions adjacently exist, so-called thermal migration is induced by which particulates in a higher-temperature region migrate toward a lower-temperature region. For example, as shown in FIG. 10, due to the thermal migration coupled with changing temperature distribution around a sensing element (insulator), the amount of PM deposited on the insulator varies and thus the output of the PM sensor varies.
Further, as shown in FIGS. 11A and 11B, when the exhaust flow speed is high, PM is unlikely to be deposited on the insulator, or, once being deposited on the insulator, is very likely to be separated therefrom. Thus, the output of the PM sensor depends on the exhaust flow speed. Separation of PM is considered to be increased as the amount of accumulated PM is increased. Therefore, the output itself of the PM sensor also influences the output of the PM sensor. Further, the inventors of the present invention also have expertise that, as shown in FIG. 12, a PM layer deposited on the surface of an insulator has electrical resistance that changes with the change of the temperature. Accordingly, for example, when the output of a PM sensor is provided in terms of a current, the output of the PM sensor depends on the temperature of the PM layer.
When the amount of PM in an exhaust pipe is desired to be obtained based on the output of a PM sensor, the influence given to the output of the PM sensor such as by the temperature of and around the PM sensor and the temperature of the PM layer is difficult to be eliminated. Thus, a scheme of appropriately correcting a detection value of the amount of PM in an exhaust pipe is desired to be developed so that the influence mentioned above is eliminated.